ALBERT

Description: The environmental drivers of zooplankton variability are poorly explored for the central subtropical Pacific, where a direct bottom-up food-web connection is suggested by increasing trends in primary production and mesozooplankton biomass at Stn. ALOHA over the past 20 years (1994-2013). Here we use Generalized Additive Models (GAMs) to investigate how these trends relate to the major modes of North Pacific climate variability. A GAM based on monthly mean data explains 43% of the temporal variability in mesozooplankton biomass with significant influences from primary productivity (PP), sea surface temperature (SST), NPGO and El Niño. This result mainly reflects the seasonal plankton cycle at Stn. ALOHA in which increasing light and SST lead to enhanced nitrogen fixation, productivity and zooplankton biomass during summertime. Based on annual mean data, GAMs for two-variables suggest that PP and 3-4 year lagged NPGO individually account for ~40% of zooplankton variability. The full annual mean GAM explains 70% of variability of zooplankton biomass with significant influences from PP, 4-y lagged NPGO and 4-y lagged PDO. The NPGO affects wind stress, sea surface height and subtropical gyre circulation, and has been linked to mid-euphotic zone anomalies in salinity and PP at Stn. ALOHA. Our study broadens the known impact of this climate mode on plankton dynamics in the North Pacific. While lagged transport effects are also evident for subtropical waters, our study highlights a strong coupling between zooplankton fluctuations and PP, which differs from the transport-dominated climate influences that have been found for North Pacific boundary currents.